#include <inc/elf.h>
#include <inc/x86.h>

/**********************************************************************
 * This a dirt simple boot loader, whose sole job is to boot
 * an ELF kernel image from the first IDE hard disk.
 *
 * DISK LAYOUT
 *  * This program(boot.S and main.c) is the bootloader.  It should
 *    be stored in the first sector of the disk.
 *
 *  * The 2nd sector onward holds the kernel image.
 *
 *  * The kernel image must be in ELF format.
 *
 * BOOT UP STEPS
 *  * when the CPU boots it loads the BIOS into memory and executes it
 *
 *  * the BIOS intializes devices, sets of the interrupt routines, and
 *    reads the first sector of the boot device(e.g., hard-drive)
 *    into memory and jumps to it.
 *
 *  * Assuming this boot loader is stored in the first sector of the
 *    hard-drive, this code takes over...
 *
 *  * control starts in boot.S -- which sets up protected mode,
 *    and a stack so C code then run, then calls bootmain()
 *
 *  * bootmain() in this file takes over, reads in the kernel and jumps to it.
 **********************************************************************/

#define SECTSIZE 512
#define ELFHDR ((struct Elf*)0x10000)  // scratch space

void readsect(void*, uint32_t);
void readseg(uint32_t, uint32_t, uint32_t);

void bootmain(void) {
  struct Proghdr *ph, *eph;

  // read 1st page off disk
  readseg((uint32_t)ELFHDR, SECTSIZE * 8, 0);

  // is this a valid ELF?
  if (ELFHDR->e_magic != ELF_MAGIC)
    goto bad;

  // load each program segment (ignores ph flags)
  ph = (struct Proghdr*)((uint8_t*)ELFHDR + ELFHDR->e_phoff);
  eph = ph + ELFHDR->e_phnum;
  for (; ph < eph; ph++)
    // p_pa is the load address of this segment (as well
    // as the physical address)
    readseg(ph->p_pa, ph->p_memsz, ph->p_offset);

  // call the entry point from the ELF header
  // note: does not return!
  ((void (*)(void))(ELFHDR->e_entry))();

bad:
  outw(0x8A00, 0x8A00);
  outw(0x8A00, 0x8E00);
  while (1)
    /* do nothing */;
}

// Read 'count' bytes at 'offset' from kernel into physical address 'pa'.
// Might copy more than asked
void readseg(uint32_t pa, uint32_t count, uint32_t offset) {
  uint32_t end_pa;

  end_pa = pa + count;

  // round down to sector boundary
  pa &= ~(SECTSIZE - 1);

  // translate from bytes to sectors, and kernel starts at sector 1
  offset = (offset / SECTSIZE) + 1;

  // If this is too slow, we could read lots of sectors at a time.
  // We'd write more to memory than asked, but it doesn't matter --
  // we load in increasing order.
  while (pa < end_pa) {
    // Since we haven't enabled paging yet and we're using
    // an identity segment mapping (see boot.S), we can
    // use physical addresses directly.  This won't be the
    // case once JOS enables the MMU.
    readsect((uint8_t*)pa, offset);
    pa += SECTSIZE;
    offset++;
  }
}

void waitdisk(void) {
  // wait for disk reaady
  while ((inb(0x1F7) & 0xC0) != 0x40)
    /* do nothing */;
}

void readsect(void* dst, uint32_t offset) {
  // wait for disk to be ready
  waitdisk();

  outb(0x1F2, 1);  // count = 1
  outb(0x1F3, offset);
  outb(0x1F4, offset >> 8);
  outb(0x1F5, offset >> 16);
  outb(0x1F6, (offset >> 24) | 0xE0);
  outb(0x1F7, 0x20);  // cmd 0x20 - read sectors

  // wait for disk to be ready
  waitdisk();

  // read a sector
  insl(0x1F0, dst, SECTSIZE / 4);
}
